Field of the Invention
The present invention relates to a power source device which supplies power to an electronic device such as an image forming apparatus including a printer, a scanner, a copier, or a multifunctional peripheral having the integrated functions of the printer, scanner, and copier.
Description of the Related Art
To form a toner image on a sheet, the image forming apparatus performs steps such as charging, exposing, developing, transferring, and fixing steps. The toner image is formed on an image carrier such as a photosensitive drum through the charging, exposing and developing steps. Through the transferring step, the toner image formed on the image carrier is transferred onto the sheet. Through the fixing step, the toner image having transferred onto the sheet is fixed on the sheet. In these steps, a high voltage is sometimes applied. The image forming apparatus generates a high voltage using a built-in power source device and supplies it to a component requiring the high voltage.
In a charger used in the charging step and a developing device used in the developing step, a high voltage formed by superposing a DC (direct current) voltage on an AC (alternating current) voltage is used. The superposing DC voltage is referred to as “superposed DC voltage” in the specification. Therefore, the power source device comprises a DC high voltage power source circuit and an AC high voltage power source circuit.
The AC high voltage power source circuit drives switching element such as a field effect transistor (FET) through, for example, a full bridge circuit to generate an AC voltage of high voltage by a transformer. A DC voltage for driving the AC high voltage power source circuit (hereinafter referred to as “driving DC voltage”) is generated from a commercial AC voltage. US Publication No. 2012/0070177 discloses a power source circuit in which a driving DC voltage is directly applied to a switching element and a voltage converter such as a transformer is used to generate an AC voltage of high voltage.
It is desirable that, when the driving DC voltage is directly applied to the switching element of the AC high voltage power source circuit, the driving DC voltage does not vary. Actually, however, the driving DC voltage varies due to some reasons including a variability caused by an individual variability of AC/DC converter generating the driving DC voltage, and use environment and use condition of the AC/DC converter. Further, in the image forming apparatus, the driving DC voltage generated by the AC/DC converter is sometimes simultaneously used in a plurality of components (loads). This also a variation factor of the driving DC voltage.
The variability and the variation in the driving DC voltage affect voltage in a primary coil of the transformer in the AC high voltage power source circuit. This results in a variation in an amplitude of the AC voltage generated in the AC high voltage power source circuit. The variation in the amplitude causes a change in a charge amount of the image carrier in, for example, the charging step. The change in the charge amount of the image carrier affects a toner adhesion amount in the developing step. This causes a variation factor of a density of the toner image, thereby the images with desired density cannot be formed. One of the solution to suppress the variation in the driving DC voltage is using a regulator. This, however, results in an enlargement of a circuit scale and an increase in cost so that this is not a decisive solution.